Dyes for organic solar cells are molecular dyes used as a sensitizer in dye-sensitized solar cells. As a method for purifying such molecular dyes, proposed has been a method for purifying a dye by adding an alkali to a sample containing a molecular dye of which the purity fails to reach a desired level, to convert the dye into aqueous form, and fractionating the dye through reversed-phase chromatography (Patent Document 1).